MicroElectroMechanical Systems (MEMS) are used in a wide variety of systems such as accelerometers, gyroscopes, infrared detectors, micro turbines, etc. For high volume applications, fabrication costs can possibly be reduced by monolithic integration of MEMS with the driving electronics. Also, for 2D imaging applications (e.g., detectors, displays) monolithic integration of MEMS and CMOS is a good solution as this simplifies the interconnection issues. The easiest approach for monolithic integration is post-processing MEMS on top of the driving electronics, as this does not introduce any change into standard fabrication processes used for realising the driving electronics. It also allows the realisation of a more compact micro-system as the MEMS device can be formed on top of the driving electronics. This is not possible if the MEMS-device is produced prior to the formation of the driving electronics. On the other hand, post processing imposes an upper limit on the fabrication temperature of MEMS to avoid any damage or degradation in the performance of the driving electronics. An overview of the several approaches with respect to hybrid integration of driving electronics and MEMS devices can be found in ‘Why CMOS-integrated transducers? A review’, Microsystem Technologies, Vol. 6 (5), p 192–199, 2000, by A. Witvrouw et al.
For many micromachined devices, such as transducers and other free-standing structures, the mechanical properties of the applied thin films can be critical to their success. For example, stress or stress gradients can cause free-standing thin-film structures to warp to the point that these structures become useless. In particular, in MEMS processing, a sacrificial layer is first deposited on the substrate, as disclosed in U.S. Pat. No. 6,194,722, entitled “Method of Fabrication of An Infrared Radiation Detector and Infrared Detector Device.” U.S. Pat. No. 6,194,722 is hereby incorporated by reference in its entirety. A second layer is then formed on the sacrificial layer. Thereafter, at least a portion (and preferably all of) the sacrificial layer is removed. The second layer is thereafter subject to bending and warping due to stresses in the second layer.
The control of stress of polycrystalline silicon (poly Si) has been widely used for MEMS applications. The main disadvantage of this material is that it requires high processing temperature, higher than 800° C., to achieve the desired physical properties especially stress as explained in “Strain studies in LPCVD polysilicon for surface micromachined devices,” Sensors and Actuators A (physical), A77 (2), p. 133–8 (1999), by J. Singh S. Chandra et al.
Polycrystalline silicon germanium (poly SiGe) seems to be an attractive alternative to poly Si as it has similar properties. The use of polycrystalline silicon germanium as a low bandgap material for electronic applications such as gate-electrode, multi-junction solar cells or thin film transistors (TFT) as used e.g. in large area electronics such as active matrix liquid crystal displays, is known in the art. For these applications the defect density, the conductivity and the crystal structure of the poly SiGe layer are of importance. The corresponding deposition methods aim at the control of the grain boundaries and might include steps to recrystallize the as-deposited layer. The presence of germanium reduces the melting point of the silicon germanium alloy and hence the desired physical properties are expected to be realised at lower temperatures, allowing the growth on low-cost substrates such as glass. Depending on the germanium concentration and the deposition pressure, the transition temperature from amorphous to polycrystalline can be reduced to 450° C., or even lower, compared to 580° C. for LPCVD poly Si.
In EP application EP 0 867 702, a method for fabrication an infrared detector device is disclosed. An infrared detector is an example of a MEMS device. As poly SiGe has a 5 times lower thermal conductivity compared to poly Si, the applicant used poly SiGe in the formation of the infrared detector. The applicant outlined the necessity to use thin films with a low, preferably tensile, internal stress. The application therefore discloses a method of controlling the stress in a polycrystalline SiGe layer deposited on a substrate such as silicon oxide, by varying the deposition pressure and/or the annealing temperature. In a preferred embodiment of the application, an RPCVD (reduced pressure chemical vapour deposition) deposition process for forming poly SiGe is disclosed. PCT application WO 00/42231 discloses the use of silicon-germanium as a sacrificial and as a structural layer to create free-standing or overhanging MEMS structures. The poly SiGe was deposited in a LPCVD system at 600 mTorr, resulting in a low compressive stress as explained in “Post CMOS Modular Integration of poly-SiGe microstructures using poly-Ge sacrificial layers,” Solid state sensors and actuator workshop Jun. 4–8, 2000 by A. E. Franke et al.